Optically variable foils, optically variable pigments (OVP), and coating compositions comprising OVP, noteworthy optically variable ink (OVI®), are known in the field of security printing. Such optically variable elements exhibit a viewing-angle or incidence-angle dependent color, and are a preferred choice to protect banknotes and other security documents against the illegal reproduction by commonly available color scanning, printing, and copying office equipment.
To enhance the counterfeit resistance and the ease of visual authentication of documents protected by optically variable security elements, it has been proposed to combine, on a same document, more than one optically variable feature. WO 2005/044583 discloses the use of a same optically variable security element in more than one constituting parts of a security document. WO 96/39307 discloses a paired optically variable device, comprising first and second optically variable devices in spaced apart positions on a same surface, having first and second optically variable pigments disposed in said first and second optically variable devices, respectively, wherein said optically variable pigments have the same color at one determined angle of incidence, and different colors at all other angles of incidence.
The device of WO 96/39307 is characterized in that the designs for the paired optically variable pigments are selected such that, in the (CIELAB) a*b* diagrams, representing the color of the said pigments as a function of the viewing or incidence angle, there are crossover points corresponding to viewing or incidence angles at which two of said optically variable pigments have the same hue. The first and the second optically variable pigments of WO 96/39307 are embodied, in the case of all-dielectric interference pigments, as different quarter-wave designs at an approximately same design wavelength. In the case of metal-dielectric interference pigments, the first and the second pigments are embodied as different half-wave designs at an approximately same design wavelength.
The principal shortcoming of the device of WO 96/39307 is that the required “same hue” of the first and the second pigment at one determined angle of incidence must be produced through different spectral characteristics, because it is impossible to realize the same spectral characteristics using different quarter-wave or half-wave designs. The observed hue merely represents the projection of the pigment's spectral reflection characteristics, i.e. the reflection intensity as a function of wavelength, onto the 3-dimensional space of human color perception, and, as known to the skilled person, different spectral characteristics can have the same projection onto the space of human color perception (color metamerism).
A consequence of this fact is that the perceived colors of the first and the second optically variable pigments used in WO 96/39307 depend in a different way on the spectral characteristics of the illumination source, such that a said crossover point, at which two said optically variable pigments have the same hue, may only be observable under a determined type of light source (e.g. incandescent light), and not appear under a different type of light source (e.g. fluorescent light).
It is the object of the present invention to overcome this shortcoming of the prior art, and to disclose a true paired optically variable security element which always exhibits color matching at a determined angle of incidence, independent of the illumination source used.